This invention relates to a piston and connecting rod assembly for reciprocating piston machines.
In reciprocating piston machines the hollow-cylindrical piston pin constitutes a force-transmitting element and a pivot of a hinge by which the piston, and the connecting rod are connected. The forces exerted on the piston pin tend to deflect and shear the piston pin and to deform it to an oval shape. For a given cross-sectional area of the piston pin, an increase of its outside diameter will decrease its deflection and will increase its deformation to an oval shape. An excessively large deformation to an oval shape may be avoided by the use of a piston pin having a larger wall thickness. In practice, efforts are made to design the piston pin so that its deflection and its deformation to an oval shape take place appropriately to the same extend. Individual dimensions of such piston pins are specified in DIN 73121 and DIN 73122. Owing to the deformation of the piston pin to an oval shape, the highest stress is a tensile stress in the bore of the piston pin. The piston pin bosses are subjected to a compressive stress at their top apex throughout their cross-section. A considerable bending load, which is due to the oval shape of the piston pin, is superposed on said compressive stress. This results in an excessively high stress on the piston pin bosses and may result in a formation of cracks in the bosses.
For this reason, highly stressed piston pin bosses can be used only in conjunction with a piston pin which has a high resistance to deformation. But this fact is inconsistent with the general trend to reduce the masses of reciprocating piston machines. Higher stresses will be permissible if a tapered piston pin is used, which is formed with two annular conical constrictions, each of which extends from a point adjacent to the inner edges of a piston pin boss to a point adjacent to the inner edge of the adjacent small end bearing. But such tapered piston pins can be provided only in shapes having rotational symmetry. This is due to the nature of the grinding operations used to machine the piston pin and to its functional requirements because the piston pin is usually mounted to float so that it will rotate in operation. For this reason such a tapered piston pin cannot be used on conjunction with a tapered to stepped boss and whereas it can compensate the longitudinal deformation, it cannot compensate the deformation of the piston pin and its bore to an oval shape in cross-section (DE-B-No. 26 15 212).